In modern military aircraft pilot ejection seats are provided to quickly free a pilot from a disabled aircraft. Usually, upon ejection a small drogue chute is deployed to slow the ejected pilot to a satisfactorily safe velocity prior to deployment of a main chute. A number of systems have been employed in the prior art for detecting the velocity of an ejected pilot. A typical system includes lever-mounted pitot tubes which extend from the head portion of an ejected seat during an initial ejection phase existing in the time interval between drogue chute deployment and main chute deployment, the latter to occur at about 250 knots. The pitot tubes of the prior art measure the dynamic pressure exerted on the pitot tube as a function of forward velocity. However, since the pressure readings of the pitot tube are altitude sensitive, it is necessary to provide an altimeter. With the benefit of a microprocessor (stored in the seat), look-up tables can handle the dynamic pressure and altitude readings for correlating these to actual seat velocity.
The disadvantages of the prior art pitot tube systems are several. In the first place the pitot tubes are moved to an operative position after deployment and the means for moving the pitot tubes consists of a rather complicated spring and linkage system. Since dual pitot tubes are provided for normal redundancy, this approach becomes costly and involves quite a bit of mechanics which decreases reliability. Further, an accurate reading of dynamic pressure requires that the deployed seat be right side up so that the inlet of the pitot tube correctly faces the forward position. It is quite common for the seat to become somewhat positionally offset after ejection. As a result, the pitot tubes will not allow the accurate computation of forward velocity.